Move is_class and default_access to struct_union_typet

[tautschnig]

A class_typet would always return true for is_class() and ID_private for
default_access(). Only when available for both a struct_typet and a
class_typet do they enable non-trivial use.

• Each commit message has a non-empty body, explaining why the change was made.
• My contribution is formatted in line with CODING_STANDARD.md.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

This PR failed Diffblue compatibility checks (cbmc commit: 15b43ae).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/85541410
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

[allredj]

Passed Diffblue compatibility checks (cbmc commit: cf07c6c).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/85542675